Many devices have been proposed which use light scattering alone or sedimentation alone or light scattering or sedimentation in conjunction with other physical methods to define particle size/mass distributions. An article by Terence Allen entitled "Particle Size Measurement", Chapman & Hall, 1974 Chapters 9, 10, 12, 14, discusses these combinations and permutations in detail. However, there appears to be only one reference in the literature to a technique employing light scattering and sedimentation in conjunction, namely an article by Alfred J. Stamm entitled "The Use of Scattered Light in the Determination of the Distribution of Sizes of Particles in Emulsions", J. Amer. Soc., 47:1582-96, (1925).
Devices are known using sedimentation and turbidimetry for particle sizing. They are called photosedimentometers.
There are several A.S.T.M. papers on particle sizing with photosedimentation, for example "A Rapid Method for the Determination of the Specific Surface of Portland Cement"; by L. A. Wagner ASTM Proceedings, vol. 33-II, pp. 553-570 (1933); "Measurements of Particle Size Distributions by Optical Methods" by R. N. Traxler and L. A. H. Baum ASTM Proceedings, vol. 35-II, pp. 475ff (1935); and "Tentative Method of Test for fineness of Portland Cement by Means of the Turbidimeter", ASTM Proceedings vol. 38-I, pp. 746-756 (1938).
The basic methods and principles described in the aforementioned papers and the literature since then relates to specific devices and applications. Turbidimetry measures the portion of the light beam passing through the sample which is not scattered; turbidimetry has a sensitive disadvantage with respect to scattering. The reason is that turbidimetry measures a relatively small decrement in a large signal, whereas scattering represents a substantial increase of light signal on top of a negligible background. The present invention exploits this sensitivity advantage. Otherwise, a scattering-based sedimentation tube has no inherent advantages over a photosedimentometer.
Instruments designed solely for light scattering photometry of fluorescene measurements such as nephelometers or light scattering photometers abound on the market today, but they have not been used in a sedimentation-scattering combination as in the present invention. The aforementioned article by Stamm describes the irradiation of a square cell containing an initially homogenous suspension from two opposite directions with unpolarized white light and the observation of unpolarized scattered light at 90.degree.. The scattered light pattern from the entire length of the cell is recorded on photographic film at various fall times then converted to an analog record with a desitometer. Analysis of the data proceeds in a manner similar to the one described in the invention described below. The chief difference in detail between it and the present invention is that a single frequency, polarized and collimated (laser) beam is used in the present invention as the incident light directed at only one position on the cell, and photoelectric detection of one polarization component of the scattered light. Both of these features contribute to increased detection resolution and sensitivity of the present invention. Because of the limited treatment of data and the different nature of the scattering substance in the Stamm device, it is not possible to make a comparison of sensitivity, detection limits, etc.
U. S. Pat. No. 4,457,624 to Goldberg et al., hereinafter referred to as the patented device, relates to a suspended sediment sensor system for determining the concentration and size distribution of particles in a fluid sample for particle sizes in the range of 50-1,000 microns. There is a need, however, for such a device which measures particle sizes in the range of 5 to 50 microns. Because of this need, the present invention was developed.
The primary differences between this invention and the patented device are the method of obtaining the scattered light from the sample, method of dispersion of the sample, the size range of the sample, the light angle at which the scattered light is detected, size of the cell and the principle of measuring a sample that a loss concentration at the focus of measurement rather than starting at zero and gaining concentration at the focus of measurement.
The detector of the present invention is set at a 90.degree. angle to the incident light, rather than at a back angle of 120.degree. as in the patented device.
The sample is thoroughly mixed in the cell and at time 0 of the measurement, the sample begins to settle out. Interrogation of the change in scattered radiation is made of the completely mixed sample with the full mixture's scttering being measured immediately and the decrease in intensity being measured as a function of time. In the patented device, interrogation of the sample is made only as the settled out particles fall a distance of 30 inches or so through a water column.
In the patented device, the scattered light signal starts at zero and is cycled through a maximum for each size range. In this invention, the signal strength starts at a maximum and diminishes as the measurement proceeds. The sample size range that this invention measures is 5 to 50 microns. The sample size range that the patented device measures is 50 to 1,000 microns. The cell size in this invention is 10 cm. The cell size in the patented device is in the order of 290 cm. The principle of distinguishing the size ranges in the present invention is to allow the smaller particles to scatter light throughout the experiment, whereas in the patented device, the smaller particles only come into the light beam and are scattered light at the end of the measurement and as time progresses, the smaller size particles are measured at the initiation of the measurement time.